docs: def build_idx_set(branch_id, start_date):
      """
      Builds a dictionary of keys based on the branch code
      """
      code_set = branch_id.split("/")
      code_set.insert(3, "Rates")
      idx_set = {
          "sec": "/".join([code_set[0], code_set[1], "Sections"]),
          "mag": "/".join([code_set[0], code_set[1], code_set[2], "Magnitude"])}
      idx_set["rate"] = "/".join(code_set)
      idx_set["rake"] = "/".join([code_set[0], code_set[1], "Rake"])
      idx_set["msr"] = "-".join(code_set[:3])
      idx_set["geol"] = code_set[0]
      if start_date:  # time-dependent source
          idx_set["grid_key"] = "_".join(
              branch_id.replace("/", "_").split("_")[:-1])
      else:  # time-independent source
          idx_set["grid_key"] = branch_id.replace("/", "_")
      idx_set["total_key"] = branch_id.replace("/", "|")
      return idx_set